As Chris alluded to, there is the legal concept of "lesser evils." If you read thru the HRS Ch. 702 (I think it was 702...I don't recall the exact statue, and I don't have time to look it up right now) it uses the analogy of driving a vehicle without working lights at night. Under normal circumstances that would clearly be illegal. However, if there was a fire at your remote home and the only way to notify the authorities was to drive said vehicle at night, the courts would look at that action as the lesser of two evils.
That being said, it doesn't mean that you won't be charged with the crime of driving a vehicle at night without lights, just that the courts would have the option of weighing the totality of the circumstances to reach a judgement. In the stated case it would probably be unlikely that you'd be convicted. The same argument could be made with regards to using a handgun not registered to you in an otherwise legitimate self-defense situation. You run the same relative risks that the judge (or jury) would find favorably (or unfavorably) for you if you were charged with unlawful possession. And, I agree with Chris that given an unambiguous and clear need to use lethal force in self-defense, the public and political backlash from such a charge of unlawful possession would likely preclude you being prosecuted. However, if the circumstances were somewhat unclear (Think Deedy) or the public perception was mixed (think Travon Martin), the risk of charges and prosecution could increase....